1. Field of the Invention
The present invention relates to scanning electron microscopes used for obtaining topography images of samples. More particularly, the present invention provides a method and system for improving the image obtained by a scanning electron microscope by optimizing the electron yield.
2. Description of the Related Art
Conventional scanning electron microscopes (SEM) are used to obtain topographic images of a sample surface to detect, for example, imperfections on the sample surface. This is accomplished by generating a probe current which is directed in a raster pattern at the sample surface. The interaction of the electrons in the probe current with the sample surface produces secondary electrons (which are released from the sample surface due to bombardment by the probe current electrons) and backscattered electrons (which are, in effect, the probe current electrons reflected by the sample surface). The secondary and backscattered electrons are referred to herein as a signal electron beam (or signal current) and is directed to an imaging detector which produces an image of the sample surface. The interaction of the electrons in the probe current with the sample also causes absorption of some of the probe current electrons into the sample or dissipation of sample electrons from the sample surface, which results in the sample becoming negatively or positively charged, respectively. Such charging has an adverse affect on the accuracy of sample surface image detection because, for example, a positively charged sample surface will capture the probe current electrons, thereby causing a dark region to appear on the sample image as a result of the lack of, or a diminished amount of, signal electrons.
An analytical tool widely used in categorizing and analyzing samples is a yield curve as shown in FIG. 1. A yield curve is a plot of the ratio of the signal electron beam and probe current with respect to the landing energy of the probe current on the sample. An ideal condition is reached for a yield value of “1” corresponding to equal values of the signal electron beam current and the probe current. As shown in curve A of FIG. 1, two landing energy values correspond to the ideal yield condition, shown as E1 and E2. The shape of the yield curve indicates a more gradual change at E2 relative to E1 such that minor variations of the landing energy proximate the E2 value result in only minor variations of the signal current. For this reason, using a landing energy of E2 to obtain an optimal topographic sample image is more desirable than a landing energy of E1.
Prior art techniques for locating the optimal energy E2 for use in irradiating samples with the probe current are qualitative and are typically performed by a microscope technician in programming a microscope so that optimal landing energy values can be preset for a variety of samples to be examined. Such qualitative techniques entail measuring the probe current strength, such as by positioning an electron detector (e.g., a Faraday cup, etc.) in a path of the probe current, and obtaining an image of the sample by receiving the signal electron beam at an imaging detector. By obtaining various images at different landing energies and/or probe currents, the images are visually compared to select the optimal image, which corresponds either to landing energy E1 or E2. By obtaining additional images at landing energies proximate the values of E1 and E2 deduction will lead to distinguishing E1 from E2 using the known characteristics of the yield curve. Once the value of E2 is ascertained, that value will then be used to examine other like samples, such as in a quality control stage of a semiconductor substrate manufacturing facility.
A problem of the prior art qualitative approach in locating a desired landing energy E2 is that although the level of the probe current is known from the use, for example, of a Faraday cup positioned in the probe current path, the signal current received by the imaging detector is not known. Thus, an SEM technician trying to locate an optimal landing energy for producing a satisfactory sample image must do so through trial and error by, for example, setting a first landing energy and obtaining an image therefrom, and then repeating the process at other landing energy values to obtain subsequent images. This procedure is not only laborious but results in a subjective determination by the technician as to what is the “best” image.